1. Field of the Invention
The present invention relates to tube swaging methods, and more particularly, to a method of hermetically swaging the extremities of straight heat exchanger tubes into the bores of spaced tube plates.
2. Description of the Prior Art
The prior art relating to the field of heat exchangers includes, as a known solution to the problem of attaching straight heat exchanger tubes to spaced tube plates, the method of inserting the tubes into the receiving bores with a small assembly clearance and of subsequently expanding the received tube portions by rolling them against the bore walls. This roll-swaging operation results in a hermetic connection between the tube end portions and the tube plates.
One shortcoming of this attachment method, which has been known for decades, is that its performance requires a considerable amount of time. Another shortcoming is the risk of damage to the tube in its region of transition between the attached tube portion, which is received inside the tube plate, and the free portion of the tube. These shortcomings of the prior art roll-swaging method are particularly disadvantageous in connection with heat exchanger applications of which heavy duty operation and high reliability is required, as is the case in nuclear reactor installations, for example. Heat exchangers for this type of application are further subject to the stringent requirement that no annular crevices between the tube plate bores and the transition portion of the tubes must be present, because the latter tend to induce the much dreaded crevice corrosion. It thus becomes necessary to extend the rolling operation to at least the inner edge of the tube plate bores, and this must be done without overstressing and thereby weakening the tube in its transition portion.
In the copending application Ser. No. 595,428, filed July 14, 1975, is disclosed a novel tube attachment method which substitutes for the roll-swaging operation a hydraulic expansion-swaging operation involving the insertion of the tube into the tube plate bore with a certain assembly clearance, following which the outer extremity of the tube is attached to the outer edge of the bore, preferably in a welding operation; an expansion-swaging core is then inserted into the bore of the tube from its outer end, the core having two axially spaced grooves holding special pressure rings, so that an annular pressure space is defined between the expansion-swaging core and the tube bore in the axial space which is delimited by the pressure rings. Inner and outer core guide portions adjoin the pressure ring grooves opposite said pressure space. A source of very high pressure, connected to the annular pressure space via radial and axial supply bores in the expansion core, makes it possible to create a very high pressure inside said pressure space, as a result of which the tube is forced to expand radially against the tube plate bore. The weld at the tube extremity, or some other suitable connection, prevents the tube from shifting axially during this expansion-swaging operation.
This hydraulic method of attaching the tubes to the tube plate bores has been found to work satisfactorily, especially when U-shaped heat exchanger tubes are used which have both extremities attached to the same tube plate. A problem arises, however, when straight heat exchanger tubes are to be attached to two spaced tube plates. We have found that under these circumstances, the expansion-swaging operation has the effect of creating a tensile stress on the free tube length between the tube plates, because the radial expansion which takes place on the swaged tube portion has a certain axial contraction effect on the tube. Being restrained axially not only by the end welds but also by the swaged engagement against the tube plate bores, the tubes are consequently subject to tensile stress even in the absence of any heat exchanger operating pressure. Depending upon the particular heat exchanger application, the operating pressure of the heat exchanger may combine itself with the tensile prestress in the tubes, with the risk of creating tension cracks, where corrosion may take place. It is therefore necessary to eliminate this tensile prestress from the tubes and to preferably even create in the tubes a predetermined level of compressive prestress.